Harry Styles invites up to 12 girls a night to his hotel on tour.
His One Direction bandmate Niall Horan, 19, says he has "stamina" and is "unbeatable" when he's trying to get a girl.
He told the Daily Star newspaper: "Harry is just unbeatable. He never runs out of stamina.
"I've lost count of all the girls he's pulled on this tour.
"He sleeps most of the day then does the gig and stays up all night with pretty girls in his room. He's a total rock 'n' roller."
The 19-year-old heartthrob's behaviour is reportedly becoming a concern, with even his father having to step in and warn him about his womanising habits.
One insider claimed Harry - whose relationship with Taylor Swift ended in January - is "sex mad" and revealed the star is "out of control".
They said: "It's incredible, he seems to have at least three or four girls in his room every night.
"He's totally out of control. Harry is totally brazen about it. He spots a girl he likes at a gig or when he's out and about, goes right up to her and invites her to his hotel.
"He's sex mad - it's no wonder Taylor Swift couldn't keep up with him and things ended badly."
The source said Harry likes to "take his pick" and has up to a dozen girls lining up on any given night.
"After nearly every gig on the tour so far he's made sure there are loads of girls lined up at his hotel so he can take his pick," the source said.
"He's had up to a dozen waiting on a single night. They all seem overjoyed to be invited back."The rest of the lads all hang out after gigs and watch movies or play their computer games. But Harry goes off with a harem of girls and you don't see him until the next day."
'The West Australian' is a trademark of West Australian Newspapers Limited 2013.
All rights reserved.
Select your state to see news for your area.